Electrical condenser



July 13, 1943. R c SPRAGUE ETAL 2,324,178

ELECTRICAL CONDENSER Filed May 23, 1940 2 Sheets-Sheet l ROBE/PT C. SPRAGUE 8 y .1 R FRANK W GOOSE), JR. IN EATO 5 A TTORNEYS.

Patented July 13, 1943 ELECTRICAL CONDENSER Robert C. Sprague, Williamstown, and Frank W. Godsey, Jr., North Adams, Mass, assignors to Sprague Specialties 00., corporation of Massach North Adams, Mass, a usetts Application May 23, 1940, Serial No. 336,822

11 Claims.

The present invention relates to adjustable condensers and will be described in connection with one of its important applications, which is their use as the variable tuning condensers for the continuous tuning of radio receivers.

The condensers so far used for this purpose are generally and almost exclusively condensers using an air dielectric, and consisting of interleaved rotor and stator plate electrodes. For tuning the respective circuit of the radio receiver, the capacity of the condenser is varied by rotating its rotor plates with respect to its stator plates, thus causing a larger or smaller portion of the two sets of plates to come into close opposition.

As a rule the condensers of the several circuits to be tuned are provided into a single assembly or gang and the capacities thereof are simultaneous- 1y varied by a single control, whereby to compensate for capacity differences between the individual condensers of the gang, as a rule one or more so-called trimmer condensers are to be provided.

Notwithstanding their general and almost exelusive use since the advent of radio receivers, such air-dielectric tuning condensers have several shortcomings.

One of these is the large size of these condensers as compared with other types of condensers having similar capacities, because of the low dielectric constant of air and the necessity of a wide spacing between the electrodes of different polarity, due to constructional and operating limitations inherent in such condensers.

A further shortcoming of air tuning condensers is that their plates are subject to nicking or bending and require very careful setting and mounting to keep them in adjustment. Also, to insure mechanical and overall temperature stability, such condenser gangs have to be very substantially built, resulting in an expensive construction.

A particular disadvantage of air tuning condensers is that they are susceptible to microphom'c" action brought about by mechanical vibration of the large and thin electrode plates thereof. Since these vibrations and the microphonics brought about thereby, fall within the audible range reception by the receiver is badly marred.

Altogether, the variable tuning condensers of radio receivers represent one of the few components, which since the advent of radio have undergone no basic change in form; whatever improvements have been made in the design and cost of this component are the result of improvements in manufacturing technique rather than to a change in fundamental design.

While in the past it has been proposed to substitute for air tuning condensers other types of condensers in an attempt to obviate the above drawbacks, the proposed tuning assemblies have not been satisfactory.

The reasons for such failure are manifold, but can be generally assigned to unsatisfactory electrical characteristics, lack of mechanical and temperature stability, lack of continuous and sufficiently wide tuning range, complicated construction and high cost.

The present invention eliminates the various shortcomings of air tuning condensers without introducing new drawbacks.

Among the various advantages of the condensers of the invention are the following:

The condensers are small in size, simple to manufacture, light in weight, and comparatively low in cost, and are mechanically rugged and resistant to wear and atmospheric conditions.

The condensers have great mechanical and electrical stability, and such stability is practically unaffected by the humidity and temperature variations which occur under the normal operating conditions of radio receivers.

The condensers can be manufactured in mass production within practical capacity tolerances, and with low minimum capacity and wide capacity range,

The condensers are free of microphonic action.

In addition, in many cases their use eliminates one or more trimmer condensers which would be required when using conventional tuning condensers and permit exact tracking of the condensers at any position in the tuning range, for example, at the minimum capacity position.

In a condenser gang made in accordance with the invention, the individual condensers consist of two coaxial conducting cylinders which form the electrodes, one of which is laterally displaceable with regard to the other, and of an interposed dielectric layer provided on one electrode.

More particularly, one of the electrodes is a cylindrical metal core provided with a thin integral dielectric coating of a material of high dielectric constant, and the other electrode is a metal sleeve which slides over the coated core.

An important feature of the invention is the close-tolerance sliding fit between the sleeve and the coated electrode, and the extremely small air gap so formed. Special constructional features of the condenser and certain steps in the process of its manufacture insure the reliable operation of the condenser notwithstanding the small air gap, and also provide for the high stability of the condensers.

For the functional interconnection, into a gang, of the individual condensers, novel constructional means are provided to permit their simultaneous tuning.

It should be well understood that while our invention will be described in connection with variable tuning condensers of the continuous type, it is equally applicable to other types of adjustable condensers, for example, to the condensers used in push button tuning by the socalled condenser substitution method.

The various novel features of our condensers and certain steps of making same will now be described in detail in connection with specific examples and with reference to the appended drawings, in which:

Figure 1 is a side view of a condenser assembly in accordance with th invention.

Fig, 2 is a plan view in section along the line 2-2 of Fig. 1.

Fig. 3 is a right side elevation of the assembly of Fig. 1.

Fig. 4 is a sectionized front view of a condenser assembly disclosing another' embodiment of the invention.

Fig. 5 is a sectionized side view along the line 5-5 of the condenser assembly of Fig. 4.

Fig. 6 is a side view in section of still a further embodiment of the invention.

Referring to Figs. 1, 2, and 3, the condenser assembly there shown comprises two condenser units l0 and H mounted, in a manner later to be described, on a metal base l2, which as a rule is the receiver chassis itself.

Condensers l0 and II are of identical construction, except for the differences specifically to be pointed out, and identical parts will be referred to by the same reference numerals.

The condensers l0 and II each comprises as one of the electrodes, a metal cylinder [3, and as the second electrode, a hollow metal cylinder or sleeve l4.

Interposed between the electrodes I3 and I4 is a dielectric layer l5, which, according to the invention, is applied as an integral coating to the surface of the inner electrode 13. The dielectric layer I5 is preferably a potassium lead silicate vitreous enamel, as disclosed in the copending application of Stanley 0. Dorst, Ser. No. 289,292 filed August 9, 1939, now U. S. Patent No. 2,290,947 issued July 28, 1942, and is cataphoretically applied to the electrode l3.

The dielectric layer may consist of other suitable dielectric materials, for example, of various kinds of porcelain vitreous enamels, or of nonvitreous dielectric materials such as cellulose acetate, Victron, and hard rubber, to which dielectric materials may be added a filler of titanium dioxide or the like, to increase the dielectric constant and/or lower the power factor. However, irrespective of the particular dielectric used, to obtain the full benefit of the invention, the dielectric layer should be integral with the inner electrode and be applied thereto in a manner as to insure its uniformity in thickness and consistency. So far we have found that such results are best assured by using electro-catapho retic deposition.

To obtain a high degree of stability, a close tolerance in capacity value, and a reproducible capacity adjustment characteristic, and to make it possible to provide for a large capacity per square inch of electrode surface, it is essential that the electrodes I3 and I4 possess smooth surfaces, be free of any irregularities, and that the outside diameter of electrode l3 and the inside diameter of electrode 14 be held to close tolerances, for example, within a tolerance range of plus/minus .0001" to .0005". In practice, as a rule, the diameter of electrodes l3 and M are held to a tolerance of plus/minus .00025. Such close tolerances can be obtained by the grinding of the outer surface of the electrode l3 and by the reaming or broachingof the inner surface of electrode [4 or by the use of drawn metal tubes for the electrodes l3 and Id.

The dielectric coating is applied to the smoothed outer surface of the electrode l3 preferably by the cataphoretic process described in the copending application of Preston Robinson et al., Ser. No. 197,692, filed March 23, 1938, whereby a thin layer of great uniformity and of even thickness throughout the length of the electrode can be deposited on the electrode 13. As a rule we prefer to also fuse the so deposited coating 15 to the electrode l3, and thereafter subject the coating to surface grinding.

The sleeve electrode l4 engages the coated electrode l3l 5 with a sliding fit, whereby an air layer or air gap I6 of very small thickness is ilfczrmed between the dielectric I5 and the sleeve A large air gap not only greatly reduces the capacity of the condenser, but we have found, deleteriously affects the stability of the condenser. It is therefore an important feature of our condensermade possible by its construction and the process steps used in its manufacture-to make the air gap extremely small and of a thickness which is only a fraction of that of the thin dielectric layer I5. We have found that the average thickness of the air layer l6 should be generally less than .0015 and as a rule we provide for a thickness of only .00025 to .0005.

The end portions |'ll| of electrode l3 are tapered. This prevents the coating during its cataphoretic deposition from building up to a greater outside diameter at the edges of the electrode than at the remaining portions. Furthermore, the tapered portions facilitate the sliding of the sleeve electrode I4 over the coated electrode l3|5 and prevent chipping of the coating I5.

To support the condensers l0 and H on the basel2, a rigid insulating strip iii of hard rubber, "Bakelite or the like, is secured by rivets 21-21 to a metal bracket 26, which in turn is secured by rivets 2828 to the base l2. According to the invention and for reasons more fully explained hereafter, one condenser is rigidly and the other condenser is flexibly supported on the base l2, It should be noted that the non-microphonic properties of the condenser assembly brought about in a manner later to be more fully described, makes it possible to use the simple mounting method herein described, and eliminates the necessity of providing special damping means such as rubber cushions or the like, as required for conventional tuning condensers.

As shown, the condenser I0 is rigidly supported by means of a threaded spindle bolt l9 which passes through an axial bore 20 of electrode l3, and at one of its projecting ends threadedly engages a threaded metal bushing 34 riveted to the strip l8. Where it enters the bore 20 the bolt I9 is provided with a nut 2|.

The condenser I I is flexibly supported from the strip II by means of a flexible member 22, for example, by a flexible wire of steel or phosphor bronze. At one end, the wire 22 is secured within a hole 31 01' the threaded end or a bolt 23, which bolt passes through an axial bore 20 or the electrode l3. At its other end the wire 22 is secured to the strip It by being soldered to a metal eyelet 25 which in its turn is riveted to strip II.

According to the invention, the capacity adjustment of the condensers I and H, which takes place by the lateral movement of the sleeve electrodes HH relative to their respective electrodes I 3-l 3, is simultaneous, this being achieved by the interconnection of the electrodes ld-H at their top by means of a metal strip 29 (see Figs. 1 and 3), soldered or otherwise socured to them. Strip 29 has two holes 3l-3l to which the two ends of a dial cord (not shown) can be attached in known manner. The strip 29 also carries a lug 30 which serves for the outside electrical connection of the sleeve electrodes l4l4.

As an extension of the electrodes i3-l3, to guide the electrodes H-H in their movement and to support them when moved in the range of minimum capacity setting of the condensers, there are provided guide cylinders 32-32. The cylinders 32 are of insulating material, for example, of hard rubber, Bakelite, Isolantite or the like, and are preferably hollow except for their ends 38 adjacent to the electrode I3. At the ends 38, the cylinders 32 are provided with axial bores 39 through which pass bolts l9 and 23 respectively, which with their heads 40 and 4| secure the cylinders 32-32 to their respective electrodes 13-!3.

The cylinders 32-32 have a smaller diameter than have the coated electrodes l3-I5, the difference in diameter being of the order of .005" to .01", with a result that the air gap between the sleeve l4 and the guide 32 is correspondingly larger than is the air gap between the sleeve and the coated electrode. This provision is to prevent binding between the guides 32-32 and the sleeves ll-H, which would otherwise occur at certain temperature ranges because of the difference in the temperature coefficients of expansion of the insulating material of the guide 32 and of the metal of the electrode I4. An air gap of the order of .005" to .01" is sufllciently large to prevent such binding, while on the other hand, it is suiiiciently small to provide for a positive guiding of the sleeves i i-l4 and to insure the exactness of the capacity setting in the range of the minimum capacity setting of the condensers.

Besides being adjustable simultaneously with condenser l I, condenser II] can also be separately adjusted by the lateral movement of its electrode l3 with respect to its electrode 14. This is achieved by turning the spindle l9 by means of a screw-driver engaging its slotted end 42. This permits the capacity value of the condensers l0 and H to be either equalized-preferably at the minimum capacity setting of the condenser-or to assume a constant difference throughout the adjustment range of the condensers.

The capacity setting of condenser l0 so obtained, is maintained by suitable locking means, for example, by a resilient U-shaped metal strip 33 which straddles the strip I8 and is secured to it at one end by means of the bushing 36, and is threadedly engaged at its other end by the spindle IS. The engagement of the strip 33 by the spindle i9 is such that the strip 33 is flexed nance of the relative position of the two con densers throughout their adjustment range, and on the other hand, safeguards against binding between the coated electrodes |3-I5 and their respective sleeve electrodes l4.

Furthermore, since each sleeve I 4 engages its coated electrode l3-I5 with a sliding lit, the sleeve electrodes mutually load and mechanically damp each other, and microphonics due to vibrations or other disturbances are prevented. This damping action oi the electrodes is so effective that it eliminates microphonics even with a vertical mounting of the assembly, i. e. with the axis of the electrodes in a vertical position. This damping efiect can be further increased by a slight but permanent distortion of the flexible member 22 which supports the coated electrode l3-l5 of condenser H.

As has already been pointed out, an important feature of the condenser of the invention is the small air gap between the sleeve electrode and the coated electrode.

The importance of such a small and uniform air gap will be appreciated if one considers that the air gap forms a series capacity with that of the dielectric coating. As the air has a dielectric constant of only 1, whereas that of the dielectric coating is much greater, for example, the potassium lead silicate above referred to has a dielectric constant of approximately 7, the effective capacity of the condenser is greatly diminished if the air gap is large.

In practice the air gap is made less than onehalf of the thickness of the dielectric coating and in general we prefer to keep it between .00025" and .0005".

A close tolerance of the air gap is essential with such a small air gap, not only to prevent binding of the sleeve electrode but also to insure stability of the condenser at all capacity values, and a linear characteristic of capacity change throughout the entire adjustment range of the condenser.

A further important feature of the condensers of the invention is the use of a dielectric which has a high dielectric constant, and which can be applied to the electrode in a very thin layer, and can also be subjected to surface grinding.

A thin dielectric layer is essential for two reasons: First. because the thinner the dielectric the greater the capacity of the condenser for otherwise identical dimensions of its elements. Secondly, the temperature coeflicient of the dielectric material being different from that of the electrodes, its difierent expansion from that of the metal of the electrodes would cause with varying temperature an important change in the thickness of the air layer.

Indeed we prefer to use for the coating a dielectric the expansion coefllcient of which closely approximates that of the metal of the electrodes;

The ability of machining. particularly surfacegrinding the dielectric, is also 01' great importance.

The vitreous dielectric material aforementioned lends itself excellently to surface grinding, and thus the coating can be ground to very exact tolerances without chipping, and assumes a perfectly smooth surface free of any irregularities as a result of such surface grinding.

The grinding, however, removes the outer glazed surface of the vitreous coating and tends to open pores and fissures exposing the underlying metal of the electrode at such spots. Within such pores and fissures moisture may be absorbed leading to attacks on the underlying exposed metal, which is usually iron, and causing a deterioration of the condenser.

To prevent this, we prefer to subject the coated and surface-ground electrode to the process described in the copending application, Ser. No. 336,821 filed May 23, 1940, of Frank W. Godsey, Jr., which process provides for local corrosion protective platings or coatings wherever the metal is so exposed by the pores.

Another important factor, which makes it possible to reliably operate the condensers of our invention notwithstanding the extremely small air gap existing between the sleeve and the coated electrode, and safeguards the exactness and stability of the capacity settings of the condenser throughout its operating range, is a processing of the condenser elements before and after their assembly into a condenser.

The processing of the sleeve electrode and of the coated electrode before their assembly, consists of the following steps:

For the removal of grease, grit and the like the electrodes are subjected to a caustic wash, for example, in an aqueous solution of sodium hydroxide, this being followed by a rinsing in water.

Thereafter the coated electrode is preferably also subjected to the processing described in the aforementioned application of Frank W. Godsey, Jr., to provide corrosion-resistant local platings or coatings on such portions of the electrode which are exposed by pores or fissures which extend through the dielectric layer.

After this the electrodes are assembled into a condenser and subjected to a special boiling process, which process constitutes an important part of the invention. This boiling in its preferred execution takes place as follows:

Theelectrodes of the assembled condenser are disengaged and the assembly is placed in a glass container containing vigorously boiling distilled water and left therein for at least one hour. The assembly is then removed from the boiling water and after shaking off the adhering water it is dried preferably by means of an air blast heated to 100 C.

It should be noted that the use of a glass or other non-corrosive container is desirable for when metal containers are used an electrochemical reaction may occur between the metal of the container and the metals of the condenser, which, however slight, may cause metal particles of the container to deposit on the condenser and deleteriously affect its performance.

The effect of the above boiling process is quite remarkable. We have found that condensers which are not so subjected are greatly affected in their stability, particularly with regard to humidity and temperature variations, whereas the condensers which are so processed exhibit a very remarkable degree of stability and their capacity is substantially unaffected by moisture or temperature changes even under very severe operating conditions.

While the effect of this boiling process is not entirely understood, it seems that it removes from the surfaces of the electrodes and of the dielectric coating as well as from the pores of the dielectric coating, all contaminations and all soluble material adsorbed in the dielectric, and the soboiled surfaces of the condenser element because of their pure and clean condition exhibit a moisture-repelling action.

It should be noted that the individual adjustability of the capacity of a condenser of a gang, with regard to the other condensers, eliminates the need of a separate trimmer condenser therefor, to permit exact tracking of the condensers at any position in the tuning range, for example, at the minimum capacity position. This is one of the added advantages of a variable tuning condenser made in accordance with the present invention.

While the condensers above described have a straight-line capacity characteristic; 1. e. the capacity change is directly proportional to the linear movement of the sleeve, other characteristics can be easily provided by simple and known nonlinear mechanical drivers.

As illustrative of the compact assembly which the invention provides, and the high capacity range of the condenser units, the dimensions of the elements and electrical characteristics of a typical assembly such as is shown in Figs. 1 to 3 are given.

Length of electrode '3 inChCS 13 2 Diameter of electrode l3 do .5620 Length of sleeve electrode l4 do 1 5 Inside diameter of sleeve electrode l4 do- .5730

Thickness of dielectric layer l5 do .005 to .00525 Maximum variation of outside diameter of layer [5 do plus/minus .0001 Dielectric constant of dielectric material 7 Average thickness of air gap l6 inch .00025 to .0005 Length of guide cylinders 32 do Outside diameter of cylinders 32 do .565

Power factor at maximum capacity setting per cent Approximately or Power factor at miniless than .012

mum capacity setting do Approximately or less than .05

Condensers comprising the above elements have a capacity range of from 6 to 450 rfs and the overall dimensions of an assembly of two of such condenser units such as is shown in Figs. 1 to 3,

A corresponding tuning condenser consisting of inter-leaved rotor and stator plates occupies approximately five times the volume of the assembly of the invention and furthermore its weight is approximately in the same proportion.

It should be well understood that instead of having only two condenser units forming the assembly, as is the case in the foregoing example, the condenser gang may consist, according to the invention, of a larger number of condenser units.

The condensers of the invention also lend themselves for pushbutton tuning or to be used as trimmer or padding condensers, whereby a high stability and wide adjustment range of such condensers is desired.

In Figs. 4 and there is shown a condenser assembly which while also usable as a variable tuning condenser, is particularly adapted for pushbutton tuning of a radio reeciver; i. e. for the tuning of the resonant circuits of a radio receiver to fixed pro-selected frequencies.

While in practice the number of sets of condensers-which corresponds to the number of frequencies, 1. e. to the number of transmitting stations which can be tuned in by the push-button-is usually four to ten, and the number of condensers per set-which number corresponds to the number of tuned circuits in the receiver-is usually two to four, for sake of simplicity the assembly shown in these figures consists only of two sets of condensers, and each set consists of only two condenser units.

Neither the circuits of the radio receiver nor the switching means to place the condensers in circuits, need to be described here as such are well known to those skilled in the art.

The assembly will be described with reference to one set only, as the sets are identical in construction.

The assembly of Figs. 4 and 5 comprises an insulating base 50 of hard rubber, "Bakelite or the like, to which is secured by means of rivets 52-52, a cover 5| open at the two sides. The cover 5| is preferably stamped from a sheet of cold rolled steel or other hard metal.

The condensers are disposed within the cover 5| with their axes perpendicular to the base 50.

Each set comprises two condenser units 53 and 54. These condensers are substantially identical with those described in Figs. 1 to 3 and each comprises a metal cylinder electrode 55, a metal sleeve electrode 56 and a dielectric layer 51 forming an integral coating, preferably of a fused vitreous insulating material, on the electrode 55. It should be here stated that the coated electrodes can be either solid cylinders except for the axial bore, and have been shown as such in Figs. 1 to 3, or may be hollow sleeves or cups and have been shown as such in Figs. 4 and 5.

As shown, the condenser 53 is rigidly supported by means of a threaded spindle bolt 59 which passes through and threadedly engages a threaded axial bore I5 of the closed end of electrode 55, and at its projecting end threadedly engages the base 50.

Condenser 54 is flexibly supported by means of a flexible wire 6|, one end of which is secured within a hole I6 of the threaded end of a bolt 63 passing through and threadedly engaging a threaded axial bore I5 of the end of electrode 55. At its other end the wire 6| is secured to the base 50 by being soldered or otherwise fastened to an eyelet 62 which in turn is rivetted to the base 50.

The sleeve electrodes 56-56 are secured together by a metal block I2 which conforms to their opposing surfaces and which is soldered to them. A bolt 'II abutting with its head 80 against the top of cover 5| passes through and threadedly engages the block I2 and passes with its free end through a corresponding hole 8| of base 50. The holes in the top of the housing 5| and in the base 50 are sufiiciently large to permit free movement therethrough of the bolt II.

The capacities of condensers 53 and 54 are simultaneously adjusted by vertical displacement of electrodes 66-," relative to their corresponding electrodes 55-55. This is effected by rotatins the bolt head 30.

Vertical displacement of the bolt 1| during such rotation is prevented by a collar I3 provided on the bolt and secured thereto at the bottom side of the cover top, for example, by means of a set screw (not shown).

For the guiding of electrodes 56-56 in their movement, electrodes "-55 are each provided with a guide cylinder 65. The cylinders -65 are shown identical to the cylinders 32-32 described in connection with Figs. 1 to 3, and are secured to the electrodes 55-55 by means of the spindle bolt 53 and the bolt 63 respectively, similarly as described in connection with the condensers of Figs. 1 to 3.

The adjustment setting of condenser 53 is maintained by a flexed resilient metal strip 68 secured to the base 50 by a. rivet 61 and which engages the spindle 53 by means of a tapped hole. An extending portion 66 of strip 68 serves as outside electrical connection for electrode 55 of condenser 53, whereas a terminal lug I0 secured by the eyelet 62 serves as outside electrical connection for electrode 55 of condenser 54.

The terminals 66 and 70 of each condenser set electrically connect to a suitable push-button selective switching device for connecting the individual condenser sets in circuit in well known manner.

Outside electrical connection to electrodes 56-56 is provided by 9. lug 14 secured to the base by one of the rivets 52.

In the two embodiments of the invention heretofore described, the condenser units are disposed with their axes in parallel. The invention also lends itself to a tandem arrangement of the units, such an embodiment being shown in Fig. 6.

The assembly shown in Fig. 6 comprises two condenser units I00 and I0| mounted on a common metal base I02 which in practice is preferably the chassis of the radio receiver to which the tuning condenser assembly is applied.

The condensers themselves have essentially the same construction as the condensers oi. Figs. 1 to 5, each condenser comprising as one electrode a metal cylinder I03 and as the other electrode a metal sleeve I04, the former being provided with a dielectric layer I05 forming an integral coating thereon.

Interposed between the electrodes i04-I 04 and the base I02 are insulating strips IIO-IIO of hard rubber, Bakelite or the like each having surfaces conforming to the opposing surfaces of electrode I04 and the base I02.

The electrodes I04 are secured to their respective insulating strip 0 by means of a bolt III whereby the base I02 is cut out at II5 around the head of bolt III so as to electrically insulate it from the base I02. Each of the strips H0 is held to the base by a bolt II 3.

Outside electrical connection for each electrode I04 is provided by a lug II4 secured to the strip IIO by the bolt III and disposed within the cut-out portion I I5 of the base I02.

Abutting against the left end of electrode I03 of condenser I00 is a guide cylinder I01 of hard rubber, Bakelite, Isolantite or the like, and is preferably hollow except for its end portion I30 adjacent to the electrode I03. Guide I01 and electrode I03 are held together by means of a stud bolt I08 passing through axial bores of electrode I03 and guide portion I30. The bolt I08 is provided at its two ends, where it enters respectively, emerges from the electrode I03 and the guide portion I30, with nuts I-I09.

Abutting against the corresponding end of electrode I03 of condenser IOI. is a guide cylinder IIB similarly of hard rubber, Bakelite" or the like and solid throughout its length except for an axial bore therein. Guide III; and electrode I03 are held together by means of a stud bolt I", which passes through axial bores of electrode I03 and guide H6, and is provided at its protruding ends with nuts IIB-I I8.

The electrodes I03-J03 are mechanically coupled by means of a flexible element, for example, by means of a flexible wire II! of steel or phosphor bronze, one end of which is secured within a hole I3I of that end of the stud bolt I08 which extends through the guide portion I 30, and the other end of which is secured within a hole I32 of that end of the stud bolt II! which extends beyond the end of electrode I03 01 condenser IOI.

External electrical connection for electrodes I03-I03 is provided by a flexible conductor I secured between the nut I09 and the electrode I 03 of condenser I00 and soldered to a lug I2I secured to base I02 by a rivet I22.

Condensers I00 and IM are simultaneously adusted by lateral movement of electr des I03-I03 with respect to electrodes I04-I04. For this purpose there is secured within a hole I33 of that end of the bolt I08 which extends beyond the electrode I03 of condenser I00, a wire hook element I23. and similarly within a hole I34 of the end of the bolt III extending beyond the guide I I6, a wire hook element I24.

A dial cord (not shown) is attached in known manner to the hooks I23 and I24.

To l mit the lateral movement of electrodes I03-403 beyond that reouired for minimum and maximum capacity positions of the condensers. a stop I25 is provided on the base I02.

The capacity of condenser I M can also be adiusted independently of that of condenser I00 by semi-permanent shifting of its electrode I03 with respect to its electrode I04. This is achieved by the simultaneous rotation, in the same direction, of the nuts I I8-I I8 which causes them, together with electrode I03, to move along the bolt I" either to the left or to the right, as may be required.

While we have described our invention by means of specific examples and in specific embodiments, we do not wish to be limited thereto for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What we claim is:

1. An adjustable condenser assembly comprising two condenser units. each of said un ts comprising two concentric cylindrical electrodes having cooperat ng surfaces free of irregularities, and a dielectric layer of uniform thickness and having a ground surface interposed between said electrodes and forming an integral coating on the inner electrode, an air gap 000%" to .0005" in average thickness interposed between the socoated electrode and the outer electrode, said outer electrodes engaging said coated inner electrodes with a sliding fit, a common mounting member for said inner electrodes, and the outer electrodes mechanically interconnected, means to damp mechanical movement between the outer electrodes and the inner electrodes and to prevent microphonic action in said condenser assembly, said means comprising a resilient member securing one of said inner electrodes to the mounting member and biasing the coated inner electrodes against the outer electrodes.

2. An adjustable condenser comprising two concentric cylindrical electrodes having cooperating surfaces free of irregularities. a dielectric layer free of surface irregularities interposed be- .tween said electrodes and forming an integral concentric coating on the inner electrode, an air gap of less than .0015" average thickness disposed between said dielectric layer and the outer electrode, and an insulating cylindrical guide member disposed adjacent to and coaxially with said inner electrode, said guide member having an outside diameter less than that of the coating on the inner electrode and said inner electrode comprising a tapered portion adjacent to said guide member.

3. An adjustable condenser assembly, comprising a plurality of condenser units, each of said units comprising two concentric cylindrical electrodes and a dielectric layer disposed between said electrodes and forming anintegral concentric coating on the inner electrode, an air gap of less than .0015" average thickness disposed between said dielectric layer and the outer electrode, and mounting means for said condensers comprising a member common to all inner electrodes, a rigid member individual to one of the inner electrodes and a flexible member for each of the remaining inner electrodes.

4. An adjustable condenser assembly, comprising a plurality of condenser units, each of said units comprising two concentric cylindrical electrodes, a dielectric layer interposed between said electrodes and forming an integral concentric coating on the inner electrode, an air gap of less than .0015"average thickness disposed between said dielectric layer and the outer electrode, and mounting means for said condensers, said means comprising a member common to all inner electrodes, a rigid member interposed between said mounting member and one of the inner electrodes, and a flexible member interposed between said mounting member and each of the remainder of the inner electrodes, means to laterally displace the inner electrodes and outer electrodes relative to each other and thereby simultaneously vary the capacity of all of the units, and means comprising said rigid member to independently adjust the capacity of its condenser.

5. An adjustable condenser assembly comprising a plurality of condenser units, each 01' said units comprising two concentric cylindrical electrodes, said electrodes having cooperating surfaces free of irregularities, an integral fused coating of a vitreous material on the inner electrode forming a concentric dielectric layer thereon and being of uniform thickness and free of surface irregularities, and an air gap of less than .0015" average thickness disposed between said layer and the outer electrode, mounting means for said condensers, said means comprising a member common to all inner electrodes, the inner electrode of one of the condensers being adjustably and rigidly secured to said member, and the remaining inner electrodes being flexibly secured to said member, a common member interconnecting said outer electrodes and means connected to said second member to simultaneously adjust the capacity of the condensers by axial displacement of their outer electrodes relative to the inner electrodes.

6. An adjustable condenser assembly comprising two condenser units. each of said units comprising two concentric cylindrical electrodes, a coated electrode into a condenser and subjecting dielectric layer of uniform thickness interposed the so-formed condenser to a treatment in hot between said electrodes and forming a concendistilled water.

less than .0015" average thickness disposed between said dielectric layer and the outer electrode, lindrical electrode free of surface irregularities,

tween said rigid member and the other inner trode, grinding the surface of the coating to electrode, means to simultaneously displace the 10 within a plus/minus .0001" variation in its outinner electrodes relative to the outer electrodes, side diameter, and thereafter subjecting said and means comprising said rigid member to indecoated electrode to a treatment in boiling dispendently adjust the capacity of its condenser. tilled water.

7. An adjustable condenser assembly compris- 10. In the manufacture of electrical coning two condenser units in tandem arrangement, l5 densers comprising as elements two electrodes each of said units comprising two concentric cyand a dielectric layer interposed between same, lindrical electrodes, said electrodes having 00- the method of stabilizing the condenser against operating surfaces free of lrregularitles, a dielechumidity eifects which comprises, sub ecting said tric layer of uniform thickness interposed beelements to a treatment in hot distilled water.

tween said electrodes and forming a concentric 20 1- An adjustable Co d s y p displace the inner electrodes relative to their 30 to the surrounding outer electrode and thereby same relative to the inner electrodes and thereby surface of a cylindrical electrode a concentric simultaneously vary the capacity f the condenser coating of a vitreous dielectric material, fusing units.

said coating to said electrode, grinding the sur- ROBERT C. SPRAGUE. face of said coating to provide same with a sur- FRANK W. GODSEY, JR. face free of irregularities, assembling the so- 

